Such plug pins are known, for example, from DE-A No. 3436634.
Highest requirements are imposed on the accuracy of the surface quality of a plug pin and on the coaxiality of its convex surface to the optical axis of an introduced LWG, in particular in plug connections for monomode LWG. Tolerance deviations of 100 nm should not be exceeded.
This applies in particular to such plug connections in which the light-emanating surfaces of the LWG of two plug pins to be coupled are situated directly opposite to each other. But also in so-called lens plugs in which the light beam emanating from the end face of the LWG of a plug pin is introduced into the LWG of an opposite plug pin via beam-expanding lenses, the coaxiality of the convex surface of a plug pin to its optical axis must be produced with high accuracy. The required accuracy of the surface quality of a plug pin can be achieved by careful chipping treatments.
It must be possible for plug pins to be inserted into an associated sleeve in a clamp-free manner in a wide temperature range. These requirements can be fulfilled only with materials which have a very low coefficient of thermal expansion, for example, ceramic or hard metal. Additionally the same material should be selected for the plug pin as for the plug sleeve.
However, materials which have a suitably low coefficient of thermal expansion are very hard and difficult to subject to a chipping treatment.
An accurate coaxial alignment of the convex surface of a plug pin to the optical axis of its LWG by a chipping after-treatment can be achieved by means of an optical lathe (compare DE-A No. 2938910 which corresponds substantially to U.S. Pat. No. 4,289,374). However, this precision method is suitable only for soft materials, for example, German silver or copper, which have a very high coefficient of thermal expansion or tend to corrode.